Two Typhoon jets from RAF Coningsby have been scrambled to escort a passenger airliner.
The Pakistan International Airways jet has been diverted to Stansted airport which is the UK's designated counter terrorism airport.
Defence sources cannot yet say why the jets were scrambled to assist. The RAF carries out around a dozen such missions each year.
The passenger aircraft has now landed at Stansted.
This Twitter user has uploaded a photograph appearing to show the Pakistan International Airways flight on the runway at Stansted Airport:
Essex Police has confirmed the plane diverted to Stansted Airport has now landed:
The Pakistan International Airlines plane that has been diverted from Manchester Airport to Stansted is carrying 297 passengers, Reuters reported.
The passenger flight (PIA709) that has been diverted is a Pakistan International Airways flight from Lahore in Pakistan to Manchester.
The Boeing 777-340ER, which is reportedly being diverted to Stansted Airport, has a capacity of 300-550 passengers.
Two RAF Typhoon jets are escorting a passenger plane from Pakistan to Stansted airport - the UK's designated counter terrorism airport.
Essex Police said its officers are responding after "an incident" occurred on a plane:
No further details were given.
The RAF has confirmed that one of its Typhoon jets was scrambled from RAF Coningsby in Lincolnshire in order to divert a passenger jet from Manchester Airport.
The aircraft, which is still in the air, is a Pakistan International Airlines passenger plane which was flying from Pakistan to Manchester, a Manchester Airport spokesman said.